Ink jet printing systems are known in which a print head defines one or more rows of orifices which receive an electrically conductive recording fluid, such as for instance a water base ink, from a pressurized fluid supply manifold and eject the fluid in rows of parallel streams. Printers using such print heads accomplish graphic reproduction by selectively charging and deflecting the drops in each of the streams and depositing at least some of the drops on a print receiving medium, while others of the drops strike a drop catcher device.
In one type of ink jet printer, the print head includes a manifold, defining a fluid receiving reservoir, to which is bonded a relatively thin orifice plate, defining the rows of orifices. The orifice plate is made of stainless steel or nickel coated with beryllium-copper and is somewhat flexible. The orifice plate is bonded to the manifold at the periphery of the orifice plate such that it bridges and closes the manifold opening leading to the reservoir. As a consequence, the orifices in the orifice plate are in direct fluid communication with the reservoir.
As fluid is applied under pressure to the fluid receiving reservoir, it flows through the orifices and emerges from each orifice as a fluid filament. The fluid filament then breaks at its tip into a succession of fluid drops. Left to natural stimulating disturbances, the filaments would break up erratically into drops of various sizes at irregular intervals. As can be appreciated, in order to provide precise charging of the drops as they are formed, it is important that the drop breakup process be uniform and that drops of substantially constant size and spacing are formed in each stream.
In order to produce such uniform breakup of the fluid filaments, it is known to vibrate the orifice plate with an electromechanical transducer, such as a piezoelectric transducer, thus producing a series of blending waves which flex the plate. These waves cause vibration, producing pressure varicosities in the fluid filaments emerging from the orifices, and resulting in drops of relatively uniform size and spacing being formed from the fluid filaments.
In one known system for creating drops of relatively uniform size and spacing, an analog phase lock loop (PLL) generator is used. However, analog stimulation generators require the use of precision components, which are expensive and sensitive to temperature changes. Also, the analog phase lock loop requires manual adjustment of the voltage controlled oscillator of the PLL by an operator.
It will be appreciated that it is desirable to precisely control the amount of vibration. A problem has been noted, however, in that the amplitude of the mechanical vibrations required for optimum stimulation has been found not to be uniform. The amplitude of the mechanical vibrations are sensitive to ambient temperature variations, and the tuned elements used to generate vibrations are subject to many variations. Thus, stimulating each of a number of orifice plates or print heads with a transducer driven at a single vibrational amplitude level results in at least some of the print heads producing jet drop streams which are either over stimulated or under stimulated. As a consequence, the ink drop breakup process lacks uniformity.
It is seen then that there exists a need for a stimulation generator which provides optimum stimulation over an extended period of operation without the need for operator adjustments.